This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Guanosine triphosphatase Ran (RanGTP) can stimulate assembly of the type V intermediate filament protein, lamin B, into a membranous mitotic spindle matrix, which is required for proper mitotic spindle assembly. We report here that the isolated spindle matrix contains known spindle assembly factors as well as signaling molecules and transcription factors essential for organismal development. The mitotic spindle matrix is both a tensile structure for MT-based spindle assembly and a framework for tethering spindle assembly factors and cell fate determinants in mitosis.